Use vertex path on one mesh to cut a hole in another mesh
Got it!



&lt;3 Blender :]

I did it using the Knife Project tool:


Load full resolution mesh
Duplicate to 'LoRes', hide the original, select the duplicate
Remesh to a low resolution
Edit Mode
Create a vertex path around ear using CTRL+Click
Copy these into a new 'cutpath' object: 
   3dView ToolShelf -> Duplicate (click twice)
   3DViewMenu->Mesh->Vertices->Separate->Selection
Object Mode -> Select 'cutpath', SHIFT+Select 'LoRes'
Edit Mode -> Knife Project
Circle Select to select bits of the ear that got missed out


x to Delete verts



OK, now I have the LoRes mesh with the ear cutout.  Now I just have to use the same 'cutpath' to cut the ear from the original mesh, and stick it in.